A structural member or beam used in suspended panel ceilings which is held fast as its ends expands when subjected to the heat of a fire, and buckles and distorts along its length if there is no provision in the beam for accommodating thermal expansion therein in a controlled manner. This distortion in the structural beam results in dropping of the ceiling panels from the suspended ceiling. Ceiling panels for suspension ceilings can be fabricated from material to act as a fire spreading retardant when in position so that the suspended panels should be retained in the ceiling as long as possible during a fire. An expansion accommodating portion may be provided in a suspension ceiling structural beam to accommodate thermal expansion in the beam in a controlled manner so that distortion along the beam's length is minimized, thereby retaining the panels in the suspended ceiling for a longer length of time to retard the spread of fire.
U.S. Pat. No. 3,388,519 discloses an expansion accommodating structural beam for suspended ceilings. The structural strength of the disclosed beam is substantially weakened by the provision of a slot extending transversely through the beam's bead. The thusly severed bead portions must bypass one another as the web is torn along perforations therein during thermal expansion in the beam. As the severed bead portions bypass one another, the flange during its bending is distorted laterally to form a zig-zag in the beam which may permit the ceiling panels to fall from the suspended ceiling.
It is therefore an object of the invention to provide an expansion accommodating portion providing for controlled thermal expansion at a predetermined location in a suspended ceiling grid beam to substantially maintain beam alignment during thermal expansion with a minimum effect on the structural strength of the beam.
It is another object of the invention to provide a suspended ceiling grid beam which has provision for controlling thermal expansion in the beam in a manner which minimizes distortion along the beam and in the ceiling grid system of such beams so that the suspended ceiling panels are retained in the ceiling grid system for an extended period during a fire.
It is yet another object of the invention to provide a suspended ceiling grid beam wherein the side walls of the beam's bead in the locale of the expansion accommodating portion are essentially freed from transverse connection at their top and bottom over a significant length to adapt the bead side walls to buckle laterally out of their respective planes as the expansion accommodating portion takes up the thermal expansion.
It is a further object of the invention to provide a suspended ceiling grid beam wherein with the bead side walls unconnected at the bead's base, means is provided to initiate buckling of the bead side walls at a predetermined location so that the bead side walls buckle laterally out of their respective planes as the flange is folded during thermal expansion in the beam.